Many of the current generation of foggers used for spraying or dispersing pesticides, defoliants, fungicides and other chemicals are known as ultra-low volume (ULV) cold aerosol generators or fog generators. Such devices normally include a prime mover such as a small gasoline-powered engine, a blower unit driven by the prime mover, a nozzle assembly, a supply tank for the chemical, and a suitable control means. The chemical is normally fed into the nozzle assembly where it is entrained in the airstream flowing therethrough and is dispersed into the atmosphere as a fog of small droplets. The droplets typically range in size from approximately five (5) to twenty (20) microns. The generators are also normally self-contained units and are removably mounted in or on a vehicle, utilizing, for example, skids or similar platforms. A typical use for such generators is to dispense insecticide as part of a mosquito eradication program.
One problem associated with known cold aerosol generators is the difficulty of ensuring that the chemical is applied in an appropriate fog and at a desired rate, despite variations in the operation of the vehicle. For example, it is important to maintain a proper average droplet size in the fog to comply with legal regulations for the particular chemical being applied and to maintain the effectiveness of the chemical fog. Legal regulations are promulgated by the U.S. Environmental Protection Agency and by state and local governments and typically limit the maximum particle size in the fog and the maximum application rate per unit area. On the other hand, it has been observed in the art that droplets below a certain size are ineffective because they often fail to engage the target plant or insect, and it has been theorized that the surface tension of the liquid droplets and the surface tension of the atmosphere directly adjacent the target somehow inhibit the smaller droplets from contacting the surface of the target. Also, in order for the chemical to have the desired effect, the chemicals normally must be applied at or above a minimum application rate per unit area. Thus, for each chemical being applied there is a desired range of droplet sizes and application rates. Furthermore, each chemical typically has an ideal or preferred droplet size and application rate.
Ensuring that the chemical is applied in a fog having droplets falling within the desired droplet size range and at a rate within the desired rate range is greatly complicated by the fact that the vehicle used to transport the aerosol generator does not travel at a constant speed, but rather the vehicle typically stops, starts, and travels at widely varying speeds. It has been known in the art to operate the blower at a constant speed, thereby delivering air at a constant pressure to the nozzle, and to vary the rate of chemical supplied to the nozzle in response to changes in the vehicle speed in order to maintain a desired application rate. Unfortunately, this simple technique is generally unsatisfactory for maintaining a desired droplet size. This is so because at a given air pressure delivered to the nozzle, a low rate of delivery of chemical results in small droplets, while a high rate of delivery of chemical results in large droplets.
Another problem associated with known aerosol generators is that of ensuring and verifying that the chemical is applied at the correct rate and is applied over the proper areas and not elsewhere. Complete coverage of the target area is necessary to provide an effective eradication program, while coverage of areas outside the target area is wasteful. Also, verifiable records of what, where, and how much chemical was applied can provide significant protection against a subsequent legal claim by another that the spraying caused damage or injury, either directly or collaterally. In the past it has been common to instruct the operator of the vehicle on where and how much chemical to apply and to rely on the operator to recall or record where and how much chemical was applied. Unfortunately, such recollections or records of the operator are prone to be less than completely reliable. This is so because the human operator can incorrectly perceive his location and the application rate at the time of spraying, can err in recording or recalling the locations and rates, or can intentionally misstate or misrecord the locations and rates of spraying. Furthermore, the amount of information to be recorded or recalled can be voluminous if any detailed record is desired.
In the art of spreading liquid fertilizer over agricultural fields from a moving vehicle it has been known to use location-sensing equipment to proactively control the spreading of liquid fertilizer. For example, U.S. Pat. No. 4,630,773 of Ortlip discloses a method and apparatus for spreading fertilizer using a digital soil map of the various soil types in the field to be fertilized. A LORAN unit is used to determine the current location of the vehicle carrying the fertilizer in relation to the map to determine the local soil type. The application of the fertilizer is then automatically controlled in response to the vehicle s location and the local soil type. While the method and apparatus according to Ortlip may have some applicability in spreading liquid fertilizer over agricultural fields, it is considered to have little applicability to spraying chemicals in a fog using a cold aerosol generator. This is so because in using cold aerosol generators, such as for spraying insecticides in populated areas, it is important that the operator retain primary control responsibility, rather than using fully automatic control, so that the operator can quickly and easily adapt to changing circumstances. For example, if it is raining it is probably prudent to discontinue spraying a pesticide fog because the fog tends to be broken up rather quickly by falling rain. Also, if children are observed chasing after the vehicle and playing in the fog, the operator should immediately halt the spraying of the fog and should admonish the children to stay out of the fog.
Accordingly, it can be seen that a need remains for a cold aerosol fog generator apparatus and method which is responsive to human control, which maintains the application rate and droplet size within desired ranges, and which records where, when and how much chemical has been applied. It is to the provision of such a cold aerosol generator apparatus and method that the present invention is primarily directed.